1. Field of the Invention
This invention relates to a method for preparing magnetite magnetic powder.
2. Background Art
Magnetite magnetic powder is typically used in a magnetic toner for an electrophotographic process including copying machines, laser printers and plain paper facsimile machines.
Prior art industrial methods for preparing magnetite magnetic powder include a wet process and a solid phase process. The method for preparing magnetite magnetic powder according to a wet process is by adding an alkali to a ferrous chloride solution to form ferrous hydroxide and passing an oxidizing gas into the alkaline solution to oxidize the ferrous hydroxide therein, thereby producing magnetite (see JP-B 35520/1974). The method for preparing magnetite according to a solid phase process is by subjecting hematite to a reducing heat treatment with a reducing gas such as hydrogen, thereby producing magnetite (see JP-A 72630/1986).
These methods are said to have the advantage that pure spinel single phase magnetite can be produced because the oxidizing or reducing atmosphere can be controlled. However, these methods have the drawback that the manufacture cost is increased because of many steps.
Meanwhile, for utilizing iron chloride in spent pickling solution resulting from hydrochloric acid pickling of steel strips in the steel making industry, it is a common practice to produce iron oxide by roasting or pyrolyzing the iron chloride in air. The roasting or pyrolyzing technique generally includes an atomizing roaster technique of atomizing a waste hydrochloric acid pickling solution from the roaster top, effecting pyrolysis in the roaster, and collecting iron oxide from the roaster bottom and a fluidized bed roaster technique of atomizing a waste hydrochloric acid solution into a fluidized bed held at a certain temperature where pyrolysis is effected and collecting iron oxide from the roaster top.
The roasting techniques have the advantages that precipitating, grinding and other steps can be omitted and the manufacture cost of iron oxide can be lowered because a spent pickling solution of iron chloride is utilized. However, since a substantial portion of iron chloride is oxidized in air so that the major phase consists of hematite, it is difficult to synthesize magnetite powder by the existing roasting techniques.
On the other hand, the following steps were proposed as a method for producing hydrogen through thermochemical decomposition of water based on iron chloride group cycling ("Development studies on the thermochemical cycles for hydrogen production", D. Van Velzen, World Hydrogen Energy Conf., Vol. 1st, No. 1, pages 8A.83-8A.108, 1976). EQU 6FeCl.sub.2 +8H.sub.2 O.fwdarw.2Fe.sub.3 O.sub.4 +12HCl+2H.sub.2( 1) EQU 2Fe.sub.3 O.sub.4 +12HCl+3Cl.sub.2 .fwdarw.6FeCl.sub.3 +6H.sub.2 O+O.sub.2( 2) EQU 6FeCl.sub.3 .fwdarw.6FeCl.sub.2 +3Cl.sub.2 ( 3)
In the technique described in this report, ferrous chloride reacts with steam to produce magnetite as an intermediate product of the process.
However, focusing the production of hydrogen in reaction formula (1), the technique described in this report studies the relationship of a reaction rate and a reaction conversion rate (hydrogen production rate) to a flow rate of steam and a reaction temperature. Except that magnetite is produced as an intermediate product, the report lacks the specific description of considerations from the aspect of magnetite production including reaction conditions and the content of magnetite phase in the intermediate product.
As mentioned above, a method of effectively producing magnetite through pyrolysis by utilizing ferrous chloride was not available in the prior art. Although the above-cited report concerning the research work of producing hydrogen through thermochemical decomposition of water based on iron chloride group cycling discloses that magnetite is produced as an intermediate product during decomposition of water, the relationship of a magnetite phase content to reaction conditions is not considered at all.